Nowadays, electronic devices such as mobile phones, tablet phones and notebook computers become essential to daily lives. Consequently, the demands on charging performance and power-saving efficacy gradually increase. After the electronic device is fully charged by a charging device, the charging device is in a no load condition. In the no load condition, the charging device is operated at a lowest voltage. Once the electronic device is charged by the charging device again, the charging device can enter a charging state quickly because of the lowest voltage.
Generally, the electronic device is charged by the charging device through a USB2.0 interface. According to the specification of the USB2.0 interface, the output power voltage in the no load condition is in the range between 4.75V and 5.25V. Consequently, the lowest voltage of the charging device in the no load condition is set to be in the range between 4.75V and 5.25V.
A conventional charging device comprises a pulse width modulation (PWM) circuit and an AC/DC converting circuit. When the charging device is in a heavy load condition or a light load condition, the PWM circuit is used to control the output voltage of the AC/DC converting circuit. When the charging device is in the no load condition, PWM circuit is also used to control the output voltage of the AC/DC converting circuit to be in the range between 4.75V and 5.25V.
The use of the PWM circuit to control the output voltage of the AC/DC converting circuit can reduce the ripple of the output voltage. Moreover, the modulation efficiency of the PWM circuit is also satisfied. Consequently, the PWM circuit is often used to control the output voltage of the AC/DC converting circuit when the charging device is in the heavy load condition.
However, when the charging device is in the no load condition, the power-saving efficacy should be taken into consideration. That is, the lowest voltage of the charging device in the no load condition is set to be in the range between 4.75V and 5.25V. In addition, the charging device in the no load condition has to achieve the power-saving purpose. When the conventional device in the light load condition or the no load condition, the PWM circuit is used to control the output voltage of the AC/DC converting circuit to be in the range between 4.75V and 5.25V. According to the control signal from the PWM circuit, the switch elements of the AC/DC converting circuit are alternately turned on or turned off. Because of the frequent switching actions of the switching elements, the charging device in the no load condition consumes a great amount of electric energy.
Therefore, there is a need of providing a charging device and a control method in order to overcome the drawbacks of the conventional technologies.